It is known in the art to provide underwater acoustic transmitting and receiving systems towed at a constant speed through the water at a constant elevation above the ocean floor while emanating a narrow fan shaped vertically oriented acoustic beam whereby successive scan passes cover adjacent narrow strips of the submerged surface. By this system there is produced a display or record of sonic echo from objects of height on the bottom. This process is enhanced when the acoustic beams are transmitted at shallow angles to the bottom. As the vehicle or platform which carries the transducer is towed along in the water a narrow beam of pulsed sonic energy is transmitted to opposite sides, usually at different frequencies. The intersection of each pulse with the bottom travels outwardly along the bottom, and return as a sonic echo from which values are displayed and recorded.
This is known as side-scan sonar, and it is employed for analyzing the sea bed in narrow juxtaposed linear strips by deducing the echo profile therein. Acoustic transmitters and receivers are carried on board a vehicle, sometimes known as a fish, which is towed along near the bottom at a constant speed and elevation. Two emitting transducers broadcast in different frequencies in opposite directions toward the ocean bottom. Each pulse covers a lateral strip on either side of the vehicle. A receiver is tuned with the transmitters and receives echoes from the strip area. Present side-scan sonar in searching for objects on the ocean floor is limited to areas of strips on either side of the vehicle's line of travel, and excludes a strip (gap) directly beneath the vehicle.
Present side-scan apparatus has the transmitter and receiver close together, making a monostatic echo arrangement. Side-scan sonar produces a "picture" of a narrow strip of the bottom. The picture is a display of the echo strengths as a function of position along the strip. The strip has a width that is determined by the beam forming properties of the receiver array and therefore lies in a plane perpendicular to the axis of the array. The display of the echoes induced by the outgoing pulse relies on shadowing effects--the pulse is blocked from reaching the bottom and thus returns any echo from objects of some size sticking up from the bottom. The resolution of the sonar determine the sizes of the objects that can be detected this way. Objects directly below the receiver/transmitter are "illuminated" from above by the source pulse, and, thus, do not produce shadows. Objects almost directly below produce very short shadows. In order to produce long enough shadows for useful imaging, the objects must be at a horizontal distance away from directly below the transmitter/receiver. The horizontal distance is typically greater than 80% of the height of the receiver/transmitter above the bottom (corresponding to a sound ray incidence angle of 70.degree. or less between the incident ray and the horizontal bottom). Thus, at 100 ft. altitude, a strip 80 ft. wide below the receiver transmitter is not "seen". This is referred to herein as a gap.